Introduction

• Fish stock assessment was developed in the temperate regions where there are very few species of fish.

• It was based on a single species

• However, such a consideration is not very applicable for tropical fisheries where we have multi-species fisheries because;

  • Different species are mixed up
  • There is very high interaction among species

• In such situation, fish populations are divided into stocks (unit stock) for management and assessment purposes

• Understanding the distribution, abundance, and methods for estimating stock parameters is important for effective stock assessment and sustainable fisheries management.

Unit Stock

• A unit stock is a population of fish, either of a single or different species, grouped together for assessment purposes which may or may not include all the fish in a stock.

• A unit stock is a group of fish that:

  • Share a common gene pool – can interbreed
  • Occupy a defined geographical area – have a specific distribution
  • Exhibit similar life history traits – growth, reproduction, mortality
  • Are subject to similar fishing pressures – exploited by the same fisheries
  • Are managed as a single entity – for conservation and exploitation, – Can be managed independently from other stocks

Determination of a Unit Stock

• Unit stocks can be determined through various biological and ecological characteristics, including:

  • Distribution patterns – Where unit stocks are located
  • Spawning behavior – Different stocks spawn in different areas
  • Population parameters – Recruitment, growth and mortality rates, morphometric differences
  • Physiological traits – Temperature at hatching, vertebrae structure, scale decorations, parasite types

Distribution and Abundance

Geographic Distribution

• Geographic distribution refers to the spatial area where a fish stock is found, including the boundaries of its range.

Key concepts:

• Fish populations are not uniformly distributed across all suitable habitats
• Distribution patterns change seasonally (spawning, feeding, wintering areas)
• Some species have discrete distributions while others have overlapping ranges
• Distribution limits are determined by temperature, salinity, bathymetry, and food availability

Geographic Distribution

Examples:

• Migratory species (salmon) move between coastal and riverine habitats
• Demersal species (bottom-dwelling) have specific depth distributions
• Pelagic species distribute widely in open water

Management implications:

• Fishing effort should target areas of highest concentration
• Seasonal closures may protect aggregations during vulnerable periods
• International boundaries may complicate management of stocks crossing borders

Spatial and Temporal Variation in Abundance

Temporal Variation:

• Seasonal changes - Abundance fluctuates with feeding, spawning, and migration cycles
• Interannual variation - Year-to-year changes in recruitment and survival
• Long-term trends - Decadal-scale changes related to fishing pressure or environmental conditions

Spatial and Temporal Variation…

Spatial Variation:

• Patchiness - Fish concentrate in certain areas (spawning grounds, feeding areas)
• Gradients - Density changes across depth, latitude, or habitat types
• Hotspots - Areas of consistently high abundance

Spatial and Temporal Variation…

Measurement challenges:

• Detecting aggregations requires spatially intensive sampling
• Small-scale variability requires high sampling effort to quantify accurately
• Temporal dynamics require time series of observations

Habitat Requirements and Factors Affecting Distribution

Abiotic Factors:

• Temperature - Most critical factor; thermal preferences vary by species
• Salinity - Freshwater, brackish, or marine preferences
• Dissolved oxygen - Required for respiration; anaerobic zones are avoided
• Bathymetry - Depth preferences related to light and pressure tolerance
• Substrate type - Bottom type affects benthic food availability

Habitat Requirements…

Biotic Factors:

• Food availability - Distribution follows food resources
• Predation risk - Avoidance of predator-rich areas
• Competition - Space partitioning with other species
• Reproductive habitat - Essential for spawning and nursery grounds

Habitat Requirements…

Habitat Quality:

• Optimal habitat supports higher population densities
• Marginal habitat may be temporarily occupied but has lower reproductive success
• Loss or degradation of critical habitat can limit population size

Environmental Influences on Stock Distribution

Physical Oceanography:

• Ocean currents transport larvae and affect distribution
• Upwelling events create productive feeding areas
• Coastal current patterns structure population distributions

Environmental Influences…

Climate and Weather:

• Climate change shifts suitable habitat ranges
• Seasonal weather events (storms, cold snaps) cause temporary distribution changes
• Long-term climate trends alter distribution boundaries

Straddling stock and highly migratory fish stock

• With 1982’s United Nations Convention on the Law of the Sea (UNCLOS), coastal nations were given the right to manage fisheries within their Exclusive Economic Zones (EEZ)
• But of course, fish don’t adhere to imaginary lines in the ocean.
• The management of two groups – straddling and highly migratory fish stocks became problem after the 1982 Convention.

Straddling stock

• Straddling stocks are fish stocks that migrate across more than one Exclusive Economic Exclusion Zone (EEZ) and the high seas
• • Straddling stocks is the stocks of fish which migrate between, or occur in both, the EEZ of one or more states and the high seas
• Fish stocks that migrate through, or occur in more than one exclusive economic zone.
• They include species such as swordfish and marlin.

Highly migratory fishes

• Highly migratory fishes are the ones that travel long distances across international waters
• These include tuna and tuna-like species such as yellowfin tuna, bigeye tuna, and skipjack.

Management of Straddling stock and Highly migratory fishes

• Management of the straddling stock and highly migratory fishes is a shared responsibility between coastal states and the international community.

• The 1995 United Nations Fish Stocks Agreement (UNFSA), an international treaty designed to further implement provisions of the 1982 Convention which;

  • Ensure the long-term conservation and sustainable use of straddling fish stocks and highly migratory fish stocks

Management…

• Sovereign responsibility must be worked out in collaboration with neighboring coastal states and fishing entities.
• Regional Fisheries Management Organizations (RFMOs) are established to manage these stocks collaboratively.
• Transboundary stock range in the EEZs of at least two countries.
• A stock can be both transboundary and straddling.